Thailand is moving ahead with plans to relocate thousands of Cambodian refugees from settlements on Thai territory to new sites on the Cambodian side of the border despite opposition from the refugees and their leaders.

Thai authorities explain the plans as a measure to improve the security of the refugee settlements controlled by noncommunist resistance groups battling Vietnam's occupation of Cambodia. The Thais say the aim is to break up large concentrations of refugees that might be especially vulnerable in event of a Vietnamese attack.

But the move also appears to reflect a toughening attitude in Bangkok toward refugees who have been living for years on Thai soil and growing concern about being left with a permanent refugee population.

The relocation plans have met some resistance from refugees, who last month held a protest and petitioned Thai authorities not to move them. Resistance leaders complain that a new site is particularly prone to malaria and cannot be defended as well by their anti-Vietnamese guerrillas.

However, the refugees and resistance leaders reluctantly accepted the proposed relocation after it was made clear to them that Thai authorities would channel the distribution of international relief aid to the new site and that the refugees would have to move there to get it.

The relocation constitutes the latest strain in relations between Thai authorities and the anticommunist Khmer People's National Liberation Front resistance group led by Son Sann. According to leaders of the front, the Thais have used their control over food aid distribution before to bring the group in line with Thai policy, notably when Bangkok was promoting the formation of an uneasy coalition of three Cambodian resistance factions.

The relocation had been expected to start Monday with the busing of refugees from a border camp of 25,000 inhabitants at Prey Chan to a new site 24 miles to the north across the frontier from the Thai village of Sanlor Cha-ngan. However, the move had to be postponed because heavy rains made the new site difficult to reach, relief officials said.

Last week about 1,200 Cambodians from Prey Chan went to the new site to begin preparing it for settlement, the officials said. They said they now expect the relocation to take place within about a month.

The refugees moved to Prey Chan after their sprawling border camp of Nong Chan was overrun by Vietnamese troops in January. The Prey Chan site is about half a mile south of the border settlement of Nong Samet, whose 50,000 inhabitants are under the control of a different faction of the Khmer People's National Liberation Front.

Thai authorities plan eventually to move the vast Nong Samet settlement 1.8 miles east to put it on Cambodian soil, officials disclosed last week. A senior official of the Thai Supreme Command acknowledged publicly for the first time that the settlement was inside Thailand. He told a refugee conference that the residents of the Nong Samet camp had lived on Thai territory for the four years since the Vietnamese invaded Cambodia, and that the border now was quiet enough for them to move back to their own territory.

Previously the settlement had been described as straddling the ill-defined border.

The location of the camps is an important factor in the Thai government's denials of repeated Vietnamese charges that Thailand harbors Cambodian "reactionary forces" as well as communist Khmer Rouge guerrillas fighting the Vietnamese-installed regime in Phnom Penh.

Besides this political aspect, a major Thai motive in relocating the camps is believed to be fears that a Vietnamese attack on the conglomeration of Nong Samet and Prey Chan, with a combined population of about 75,000, would cause a massive and unmanageable exodus of refugees into the interior of Thailand.

For their part, leaders of Son Sann's front fear that spreading the settlements out too much will stretch their forces thin and make the camps harder to defend.

Another reason for their reluctance to move is the development by the Prey Chan resistance leader, Chea Chut, and his followers of a lucrative black-market trade in that sector of the border. Thai authorities now are understood to be willing to allow some of the Prey Chan inhabitants to remain in the area, so that the number to be relocated is expected to fall short of the settlement's entire 25,000 population.

The continuing hazards along the border were illustrated last week by a surprise Vietnamese shelling of the resistance settlement of O Bok on Cambodia's northern border with Thailand. According to relief officials, about 1,600 refugees filed into Thailand and 10 wounded were evacuated to a Red Cross hospital at Kap Choeng after about 100 shells fell on the camp.

Against this backdrop, the Thai government appears to be growing increasingly weary of the refugee problem.

In a July 7 speech to a refugee conference in Bangkok, the head of Thailand's national security council and leading refugee policy-maker, squadron leader Prasong Soonsiri, said the country currently harbors nearly 158,000 Indochinese displaced persons in addition to the 200,000 Cambodians living along the Thai-Cambodian border.

He expressed grave concern about a sharp drop in the number of Indochinese refugees leaving Thailand for resettlement abroad and accused western countries of hypocrisy by holding the Thais to a "humanitarian principle" on refugee matters while applying an "immigration principle" to their own refugee admissions.

He warned, "we in Thailand cannot uphold the humanitarian rule much longer while the rest of the world has begun to lose faith in it."

In Washington, meanwhile, the Reagan administration is preparing new guidelines on refugee processing designed to assist immigration officers in determining who qualifies for refugee status, U.S. officials here said. The guidelines will have input from the State Department, Justice Department and Central Intelligence Agency and are expected to be issued this month, the officials said.

By providing more complete information on conditions in the applicant's country of origin and the likelihood of persecution there, the new instructions may help speed refugee processing and improve its consistency, officials said.

However, they cautioned that the guidelines would not necessarily mean increased admissions of Indochinese refugees into the United States.

The limit for admissions of Southeast Asian refugees this fiscal year is 64,000, but U.S. immigration authorities have been taking in fewer applicants, and the final figure for the year is expected to be closer to 40,000.